1. Field of the Invention
The present invention relates to a drive control device which is provided with an electronic automatic transmission and an automatic drive system.
2. Description of the Related Art
An example of an prior art drive control device is disclosed in Patent Published Application No. 58-121713/1983. The drive control device in the Patent application includes a vehicle speed signal generating circuit which outputs a signal corresponding to the speed of a vehicle having an automatic transmission, a vehicle speed setting device which sets up speed of the vehicle to a constant value, a set speed signal output circuit which outputs the set speed signal when the speed is set by the vehicle speed setting device, and a comparator which compares the signal from the set speed signal output circuit and the signal from the vehicle speed signal generating circuit to output a signal corresponding to the difference between the two. The drive control device further includes a throttle valve control device which controls the opening and closing of the throttle valve corresponding to the output signal from the comparator. The drive control device is further provided with a speed maintenance detection circuit. The circuit compares the value that is set by the gear position of the automatic transmission corresponding to the vehicle speed set by the vehicle setting device and the value of the output signal of the vehicle speed comparator. The circuit outputs a signal when the output signal value of the comparator exceeds the set value. A timer outputs a signal for a predetermined period based on the output from the speed maintenance detection circuit. A shift-down circuit shifts down the gear based on the logical sum of the output signals from the speed maintenance detection circuit and the timer.
In this prior art device, when the load on the engine is increased, for example, in climbing up a slope during a constant speed drive, the gear of the automatic transmission is shifted down by a shift-down signal. Thereby, a reduction in the vehicle speed is avoided and the vehicle speed is constant. At the same time, the shift-down signal is kept for a predetermined period. This means that the hunting of the vehicle is prevented.
However, in such a drive control device, all three of the controls are executed separately. Namely, the control for opening and closing of the throttle valve by means of the automatic drive system, the control of the amount of the throttle valve opening, and the speed control by the automatic transmission based on the vehicle speed are executed separately, independent of each other. Because of this, when a vehicle climbs up a slope with the automatic drive system switched on, the automatic transmission shifts down the gear with opening of the throttle valve by the automatic drive system, in accordance with the regular speed shifting schedule. But the vehicle is kept in an accelerating state for a predetermined period during which the shift-down signal is preserved by the action of the timer. Therefore, when the vehicle reaches the top of the slope prior to the set time of the timer, the throttle valve is kept open in spite of the decrease of load on the engine. This causes an undesirable situation in which the vehicle speed is increased excessively, resulting in an overshooting accelerating state.